The patient's syncope with a slow, irregular pulse is most consistent with complete heart block (Stokes-Adams attack). In this condition, the atrial impulses fail to conduct to the ventricles, causing a significant drop in ventricular rate and cardiac output. The resulting transient cerebral hypoperfusion manifests as syncope. The pulse is characteristically slow and irregular due to the escape rhythm. This contrasts with vasovagal syncope, which generally features a normal or slow pulse without conduction abnormalities. Recognition of this bradyarrhythmia is critical as it may require a pacemaker to prevent recurrent syncopal episodes.

Reference: Braunwald's Heart Disease, Douglas P. Zipes, 11th Edition.